Electroluminescent (EL) devices are used in display devices and solid-state lighting (SSL) lamps. EL displays employ both active-matrix and passive-matrix control schemes and can employ a plurality of subpixels. Each subpixel contains an EL emitter and a drive transistor for driving current through the EL emitter. The subpixels are typically arranged in two-dimensional arrays with a row and a column address for each subpixel, and having a data value associated with the subpixel. Subpixels of different colors, such as red, green, blue, and white, are grouped to form pixels. EL lamps can employ constant- or alternating-current or voltage drive schemes. They can include a single, large area EL emitter operated at a low voltage, a plurality of small area EL emitters arranged in series so that the lamp is operated at a high voltage, and other configurations known in the art. EL devices can be made from various emitter technologies, including coatable-inorganic light-emitting diode, quantum-dot, and organic light-emitting diode (OLED).
EL emitters use current passing through thin films of organic material to produce light. In an OLED emitter, the color of light emitted and the efficiency of the energy conversion from current to light are determined by the composition of the organic thin-film material(s) used and the conditions under which it the device operated, such as the current density through the material. Different organic materials emit different colors of light. However, as the emitter is used, the organic materials in the emitter age and become less efficient at emitting light. This reduces the lifetime of the emitter. Different organic materials layered in a single emitter can age at different rates, causing differential color aging and a device whose white point varies as the device is used. The rate at which the materials age is related to the amount of current that passes through the emitter, which, in turn, is related to the amount of light that has been emitted from the emitter. Various techniques to compensate for this aging effect have been described.
U.S. Pat. No. 6,414,661 B1 by Shen et al. describes a method and associated system to compensate for long-term variations in the light-emitting efficiency of individual organic light-emitting diodes (OLEDs) in an OLED display by calculating and predicting the decay in light output efficiency of each pixel based on the accumulated drive current applied to the pixel. The method derives a correction coefficient that is applied to the next drive current for each pixel. This technique requires the measurement and accumulation of drive current applied to each pixel, requiring a stored memory that must be continuously updated as the display is used, and therefore requiring complex and extensive circuitry.
US Patent Application No. 2002/0167474 A1 by Everitt describes a pulse width modulation driver for an OLED display. One embodiment of a video display includes a voltage driver for providing a selected voltage to drive an organic light-emitting diode in a video display. The voltage driver can receive voltage information from a correction table that accounts for aging, column resistance, row resistance, and other diode characteristics. In an embodiment, the correction tables are calculated prior to or during normal circuit operation. Since the OLED output light level is assumed to be linear with respect to OLED current, the correction scheme is based on sending a known current through the OLED diode for a duration of time sufficiently long to permit the transients to settle out, and then measuring the corresponding voltage with an analog-to-digital converter (A/D) residing on the column driver. A calibration current source and the A/D can be switched to any column through a switching matrix.
U.S. Pat. No. 6,995,519, by Arnold et al., teaches a method of compensating for aging of an OLED emitter. Yet another method for aging compensation is described in US 2010/0156766 by Levey et al. The disclosure of both of these ('519 and '766) is incorporated herein by reference.
US Patent Application Publication No. 2009/0189530 by Ashdown et al. describes feedback control of RGB LEDs by superimposing AM modulation on the PWM drive signal. However, the AM modulation does not provide control of chromaticity or luminance. It serves only to differentiate the R, G and B channels when sensed by a single photosensor. It is not applicable to single-color systems such as an EL lamp with only white broadband EL emitters.
US Patent Application Publication No. 2008/0185971 by Kinoshita describes adjusting current density and duty cycle of an EL emitter independently to vary chromaticity while keeping luminance constant. However, this scheme does not perform any compensation, for aging or otherwise.
US 2009/0079678 describes a technique for reducing power consumption of an OLED by reducing drive signal, and therefore panel luminance, if an image is displayed that does not contain information in the shadow region of the tonescale.